rtc: pl031: fix set_alarm by adding back call to alarm_irq_enable

Commit c8ff5841 ("rtc: pl031: switch to rtc_time64_to_tm/rtc_tm_to_time64")
seemed to have accidentally removed the call to pl031_alarm_irq_enable
from pl031_set_alarm while switching to 64-bit apis.

Let us add back the same to get the set alarm functionality back.

Fixes: c8ff5841 ("rtc: pl031: switch to rtc_time64_to_tm/rtc_tm_to_time64")
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200714124556.20294-1-sudeep.holla@arm.com